1. Field of the Invention
The invention relates to a grinding machine having a grinder head and a method of manufacturing a semiconductor device by using the grinding machine, and specifically, relates to a grinder head for grinding a back surface of a semiconductor wafer on which semiconductor elements are formed and a grinding machine having the grinder head.
2. Description of the Related Art
In recent years, with the progress of miniaturization of electric devices, semiconductor chips, which are mounted thereon, are also miniaturized. Specifically, it is required that the thickness of a semiconductor chip should also get thinner. For example, the size of a passive device, such as a condenser being mounted on a mounting board, is changed from 1005 to 0603, and then to 0402. For this reason, an active device (such as a semiconductor device having transistors), which is mounted together with the passive device, is also desired to be miniaturized to the same degree as the passive device.
One of the methods to make a semiconductor device thin is to grind a back surface of a semiconductor wafer, as shown in Japanese patent publication 2002-301645. A general grinding machine used in this field includes a chunking table having a plurality of minute openings and a grinder head having a plurality of grindstones, which are aligned along the periphery of the grinder head. A semiconductor wafer to be ground (hereinafter called “the workpiece”) is mounted on the chunking table in the condition that the back surface of the workpiece is exposed. Then, the grindstones on the spinning grinder head contact the back surface of the workpiece, and the workpiece is ground from its back surface.
As shown in FIG. 7 of the cited Japanese patent publication 2002-301645, there are some slits created between the grindstones for the purpose of discharging a coolant (ex. pure water). For this reason, while grinding, there are two conditions at the periphery of the workpiece; that is, the first condition is that the workpiece contacts the grindstones, and the second condition is that the workpiece does not contact the grindstones. In other words, in the first condition, the workpiece is held down by the grindstones, and in the second condition, the workpiece is not held down by them. These conditions occur alternately.
When the workpiece is manufactured by a process of WCSP (Wafer-level Chip Size Package), a step difference is created on the workpiece at the periphery because a resin for sealing the semiconductor device is not formed there. Since the step difference is generally around 100 μm height, it is difficult to eliminate the step difference by a grind tape. Thus, when the workpiece manufactured by the WCSP process is affixed by the grind tape on the chunking table, a gap is formed between the workpiece and the chunking table. This means that the entire surface of the workpiece is not chucked, and the workpiece at its periphery is in a condition of floating from the chucking table.
Under this condition, when the grindstones pass on the periphery of the workpiece intermittently, vibration may occur on the workpiece at its periphery. For this reason, when the workpiece is ground from its periphery to its center, large numbers of linear scratches having a 100 μm depth are formed at the periphery of the workpiece, or the workpiece is sometimes cracked at its periphery. In the contrary case that the workpiece is ground from its center to its periphery, the workpiece at the periphery is ground more than that at other areas. As a result, the stiffness property of the workpiece at the periphery is weakened so that the workpiece is cracked or divided at its periphery in later processing.
Further, if the workpiece, which is manufactured without any step deference on its surface, is ground, the periphery of the workpiece placed on the chucking table does not float. However, the chucking area of the chucking table is generally smaller than the workpiece in the grinding machine of the related art. Thus, even if the workpiece has no step difference, the periphery of the workpiece is not affixed to the chucking table. Thus, in a case that the workpiece is ground to be relatively thin, such as less than 100 μm, the stiffness of workpiece itself is weakened so that the periphery of the workpiece vibrates more intensely. As a result, as well as the workpiece having a step difference, large numbers of linear scratches are formed at the periphery of the workpiece, or the workpiece is cracked at its periphery. Further, the workpiece is ground more at its periphery than at other areas.